1. Field of the Invention
The present invention relates generally to a method of and apparatus for heating a commodity to be heated in a vacuum chamber. Such methods are used e.g. for a degassing, soldering, sintering, hardening and in connection with coating or ion treatment methods. Thereby, an as uniform as possible heating of the commodity to be heated must be obtained.
2. Description of the Prior Art
In generally known vacuum heating furnaces the commodity to be heated is surrounded by heating surfaces from which the heat is transmitted onto the commodity to be heated by convection or heat conduction. Electrically conducting commodities to be heated can also be heated by induced current flows, and, furthermore, the use of a glow-discharge as the heat source, is known which in case of the so-called anomal discharge covers the entire cathode surface uniformly such that the commodity to be heated, which is connected as cathode, can be heated uniformly.
It is known further to heat metals to be melted in a vacuum by means of an electron bombardment. It must thereby be assured, however, that by means of a specific geometric arrangement of the source of electrons that the desired distribution of temperature on the commodity to be heated is arrived at. Until now, it was possible to reach a uniform heating only with a correspondingly high expenditure. Usually, however, the electron bombardment is applied precisely in the opposite sense, namely to produce a locally confined hot spot with high temperature differences relative to the surroundings, for which task electron beams are specifically suitable, due to the fact that they can be focussed quite easily.
A specific form of heating using electron bombardment involves a low voltage arc; in the context of this description a low voltage arc is to be understood as a gas discharge which burns between a hot cathode which emits electrons by a glow emission, and an anode (in this context it is unimportent if the cathode is maintained at the emission temperature solely by the gas discharge or if it is heated additionally). In most cases a noble gas is fed close to the cathode, e.g. into the hollow space of a hollow cathode or into a specific glowing cathode chamber which communicates with the vacuum chamber via an opening. It is a general procedure to focus the plasma entering the chamber from the hollow cathode or from the glowing cathode chamber via the opening by means of a magnetic field. The electrons travel thereby along tight or confined helical paths, of which the center lines correspond to a large extent to the lines of flux of the magnetic field. Arrangements of this kind are disclosed e.g. in the U.S. Pat. Nos. 3,210,454 and 4,197,175. Both describe the heating of a melting charge which is connected as anode by means of a magnetically focussed low voltage arc which is aimed at the melting charge. The magnetic lines of flux--and accordingly the helix-shaped paths of the electrons--extend through the mentioned opening and through the melting charge. Accordingly, the low voltage arc is here used for the production of locally limited hot spots with high temperature differences relative to the surroundings. To use electron beam furnaces or light arc furnaces for the heat treatment of commodities where the surface is to be heated uniformly, is difficult, because it was hardly possible to achieve a sufficiently uniform distribution of the density of the current on the commodity to be heated.
In accordance with the Swiss Specification CH-PS 658 545 there is, furthermore, proposed a method of a uniform heating of a commodity to be heated in a vacuum chamber by maintaining a magnetically focussed low voltage arc discharge. The magnetic field is thereby maintained in such a manner that the magnetic lines of flux extend substantially parallel to the surfaces of the commodities to be heated. This method has, however, not proven itself as satisfactory specifically for substrates or substrate recipients of rather large length, because the substrate is heated at the so-called center of the bulge or special variation of the magnetic field, more strongly than at the edge areas of the magnetic field, which leads to a loss of hardness of the base material.
The German Specification DE-OS 38 29 260 suggests, finally, to arrange during the heating-up phase a screen between the cathode and the object to be coated which is removed after reaching the process temperature. However, this procedure again does not solve the problem of uniform heating.